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I have this question which really puzzles me.

I'm from Europe and used to drive manual. I used to shift to neutral going down the hill to save on gas. Now I moved to the USA and I have cars that use automatic transmission but I was still thinking that I can use Neutral when going down the hill.

I have Honda Odyssey(05) and Ford Taurus(10).

When driving down the hill Honda I tend to use neutral. The idles go down as it should. No problem here.

When driving Ford in neutral the idle speed tends to go into higher cycles based on how fast the car is moving down the hill. This confuses me as it makes the meaning of using natural pointless. This was also happening with my old Ford. I thought it was a defect and thought when I buy a new Ford it will be OK. Any idea why Ford accelerates idle speed in neutral based on how fast the car is moving?

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    Not an answer, but this question will be an interesting read. Shifting to neutral down hill is illegal in many areas (if not most), and doesn't save much, if any, gas. It's not a good habit.
    – JPhi1618
    Commented May 2, 2016 at 14:39
  • Shifting to neutral isn't technically illegal in Europe, but it's very bad practice, as you're not in control of the car at that point. (ex-IAM member)
    – PeteCon
    Commented May 2, 2016 at 15:42
  • @JPhi1618, it is interesting to read the comments below. People have a different opinion. Some say it's good some it's not. But they mostly talk about cars with manual transmission.
    – Grasper
    Commented May 2, 2016 at 16:51
  • @Pete, not in control? That's like saying I'm also not in control when changing the gears. Sometimes in the manual car I hold the clutch for longer period of time as needed. Does it mean I'm not in control?
    – Grasper
    Commented May 2, 2016 at 16:53
  • @JPhi1618, I was referring to this mercurynews.com/mr-roadshow/ci_25868854/…
    – Grasper
    Commented May 2, 2016 at 16:58

1 Answer 1

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Ford may be trying to match the engine speed to the transmission shaft speed. This would tend to reduce wear on the clutches. When the transmission is shifted back into gear after a down hill run in neutral the transmission has to match the engine to the wheel speed. Depending how different these are clutch wear can be significant. Note that modern fuel control strategies often can turn off fuel supply to the engine in coast mode as long as the transmission is left in gear. In neutral it must supply fuel to keep the engine turning. So the neutral downhill strategy can, in some cases, use more fuel.

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  • oh ok, so there is a reason behind it and not some sort of a defect. Thanks.
    – Grasper
    Commented May 2, 2016 at 16:42
  • Even in old fuel injection systems, it is still more efficient to leave it in gear. Idling an engine is inherently inefficient. I've tested this with my 87 Daytona with a MPG readout on the dash on long hills.
    – rpmerf
    Commented May 2, 2016 at 19:08
  • @rpmerf, that doesn't make any sense. It is inefficient if you sit in one place but not if you drive miles only on idling.
    – Grasper
    Commented May 3, 2016 at 11:43
  • Idle is the least efficient - efficiency is about how much useful action is obtained from how much energy. It may use less fuel to idle than to drive, but driving gets more action from the fuel. Not your actual question, but I felt like adding my 10 cents worth!!
    – Bevan
    Commented May 4, 2016 at 8:50
  • Oh, and don't use neutral down Hills - if you use your brakes too much they will overheat and fail. In a fuel injected car like this, deceleration actually cuts the fuel altogether - you'll use less fuel leaving the car in gear!
    – Bevan
    Commented May 4, 2016 at 8:51

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